The west coast of Scotland is at its best in late spring (May to June) and early autumn (September). The days are long, the midges are less active than in July and August, and the ferry services run full summer schedules from May through September. July and August are the busiest months — the ferries fill up quickly and accommodation needs to be booked well in advance. Winter is quiet but the weather is challenging; many island services reduce frequency and some tourist operations close.
The wettest months are October through January; the drier period runs from March through June. That said, the west coast gets weather from the Atlantic in all seasons, and a clear day in February can be more beautiful than a grey one in August. Pack waterproofs regardless of when you go.